Electro-chemical measuring sensor with a potential-free sensor element and method for producing it

ABSTRACT

An electro-chemical measuring sensor (10) for determining the oxygen content of gases, in particular for determining the oxygen content in exhaust gases of internal combustion engines, with a potential-free disposed sensor element (14). The sensor element (14) has an oxygen-ion-conducting solid electrolyte body (23), preferably in the shape of a pipe or tube closed at one end, which has an exterior electrode (25) disposed on the exterior surface with a strip conductor (27) on the side toward the contact and also extending on the exterior surface, and which is inserted by means of a sealing ring (20) in a metal housing (11). The sensor element (14) has an electrically insulating layer (21) at least in the area of the sealing ring (20), which covers at least the strip conductor (27) in the direction toward the housing (11). The insulating layer (21) is formed from a mixture of a crystalline, non-metallic material and a glass-forming material. In the course of production the insulating layer (21) is subjected to a thermal treatment above the melting temperature of the glass-forming material, in the course of which the insulating layer (21) forms a glaze filled with the crystalline, non-metallic material.

BACKGROUND OF THE INVENTION

This application is a 371 of PCT/DE94/00791 filed Jul. 9, 1994.

The invention is based on an electro-chemical measuring sensor fordetermining the oxygen content of gases, in particular for determiningthe oxygen content of exhaust gases of internal combustion engines,including a potential-free arranged sensor element having anoxygen-ions-conducting solid electrolyte body, preferably in the form ofa pipe closed at one end, and electrodes with electrically conductingconnectors, with the sensor element being inserted with a sealing ringinto a metal housing and with at least one electrically conductingconnector facing the housing being electrically insulated with respectto the housing by an electrically insulating layer in the area of thesealing ring. Electro-chemical measuring sensors are embodied, forexample, in the so-called finger construction, wherein a solidelectrolyte body is sealingly fixed as a closed pipe in a metal housing.Among the finger sensors, a differentiation is made betweenpotential-free and potential-bound measuring sensors. Withpotential-bound measuring sensors, the trace of the exterior electrodeis brought into contact with the housing by means of an electricallyconducting sealing ring. With potential-free measuring sensors, eachelectrode connection is directly supplied to a control device, so thatno electric contact with the housing is permitted. In both cases a sealbetween the solid electrolyte body and the housing must be realized.

A potential-free measuring sensor is known from DE-OS 25 04 206, whereina plurality of electrically insulating, ceramic seal rings made ofsintered corundum with >90% Al₂ O₃ are employed, and which provide ahermetically sealed, electrically insulating connection between thesolid electrolyte body and the metal housing. Such a seal isstructurally very elaborate and also relatively prone to risks becauseof the multiple parallel seal with three sealing rings.

It is furthermore already known from DE-OS 26 19 746 to cover the traceon the solid electrolyte body with glazing, in particular in the areasof lower temperature.

SUMMARY AND ADVANTAGES OF THE INVENTION

In contrast to the known electro-chemical measuring sensors fordetermining the oxygen content of gases, the measuring sensor inaccordance with the invention includes a potential-free arranged sensorelement having an oxygen-ions-conducting solid electrolyte body,preferably in the form of a pipe closed at one end, and electrodes withelectrically conducting connectors, with the sensor element beinginserted with a sealing ring into a metal housing, with at least oneelectrically conducting connector facing the housing being electricallyinsulated with respect to the housing by an electrically insulatinglayer in the area of the sealing ring, and with the insulating layerbeing formed of a mixture of a crystalline, non-metallic material and aglass-forming material such that a glaze filled with the crystalline,non-metallic material is formed by heating. This sensor according to theinvention has the advantage that sealing elements which are electricallyconducting can be used for sealing the sensor element in the housing,for example a metal sealing ring or a graphite sealing ring or graphitepackage. By employing these compact seals, exhaust gas, water and/orfuel are prevented from reaching the interior of the sensor element. Theinsulating layer has a great mechanical sturdiness in respect topressure peaks which are created by the sealing ring in the course ofthe joining process. The method of the invention has the advantage thatit can be integrated into the manufacturing process of sensor elements.The application processes of the insulating layer are possible by meansof proven technology, for example, rolling on, spraying a suspension,flame spraying, plasma spraying, printing or the like.

Advantageous further developments and improvements of the measuringsensor in accordance with the invention, and of the method of theinvention are possible by means of the steps recited in the dependentclaims. A particularly good electrical insulation is achieved if theelectrically insulating layer is made from an oxide-ceramic material andan earth alkali silicate. A glaze filled with ceramic is produced fromthe mixture by means of a thermal after-treatment.

To prevent the introduction of the glass-forming material into thematerial of the electrical connection, it is practical to dispose anintermediate layer, which preferably consists of the material of thesolid electrolyte body, under the insulating layer, at least in the areaof the electrically conducting connector. The material of the insulatinglayer offers a great insulation resistance at high applicationtemperatures in comparison with the layers of the solid electrolytematerial. The raw materials used are cheaply available.

To prevent or reduce pressure peaks of a sealing element, for example, ametal sealing ring, on the insulating layer, it is furthermoreparticularly advantageous to provide the insulating layer with a coverlayer at least in the area of the sealing ring. The formation offissures in the insulating layer is prevented thereby, which otherwisenegatively affect the insulating effect and sturdiness of the insulatinglayer. The cover layer used furthermore acts as a diffusion barrier forinterfering cations, for example heavy metal cations such as Cu⁺, Cu²⁺,Fe²⁺ which emanate from the sealing element (for example a Cu-coatedsteel sealing ring) and can cause a defined electrical conductivity inthe insulating layer and in this way can destroy the insulating effectat least at high temperatures.

The course of the process can be integrated into the manufacturingprocess particularly efficiently by co-sintering the insulating layer orthe further applied layers with the solid electrolyte body. Theinsulating layer has in addition excellent adhesion which comes about inparticular by co-sintering. A greatly matched thermal expansion of theinsulating layer to the material of the solid electrolyte body has apositive effect on the layer adhesion in addition. Furthermore, thedense insulating layer protects the solid electrolyte body againsthydro-thermal attacks, in particular in the low temperature range (150°to 300° C.). The structural stability of the solid electrolyte body isimproved by this.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are represented in the drawingswherein FIG. 1 shows a longitudinal section through the portion of themeasuring sensor on the exhaust gas side, and FIGS. 2, 3 and 4 showexemplary embodiments of an enlarged sealing zone X in accordance withFIG. 1.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The electro-chemical measuring sensor 10 represented in FIG. 1 has ametal housing 11 which has a key hexagon 12 and a thread 13 on itsoutside as fastening means for the installation of a pipe for the gas tobe measured, not shown. The housing 11 has a longitudinal bore 18 with aseal seat 19 supporting a sealing ring 20. A sensor element 14 with ashoulder 16 embodied on a bulge-shaped head 15 rests on the seal seat 19provided with the sealing ring 20. A sealing face 22 is formed on thebulge-shaped head 15 of the sensor element on the side toward the sensorelement between the sealing ring 20 and the sensor element 14. The sealseat 19 itself forms a sealing face on the side toward the housing. Thesealing zone X being formed on the sealing ring 20 is shown in anenlargement in FIGS. 2 to 4.

In the instant example, the sensor element 14 is an oxygen sensor, knownper se, which is preferably used for measuring the partial oxygenpressure in exhaust gases. The sensor element 14 has a tube orpipe-shaped solid electrolyte body 23, whose end section toward the gasto be measured is closed off by means of a bottom 24. A layer-like,gas-permeable measuring electrode 25 is disposed on the outside of thesolid electrolyte body 23 which is exposed to the gas to be measured,and a gas-permeable and layer-like reference electrode 26 is disposed onthe side facing the interior and is exposed to the reference gas, forexample air. The measuring electrode 25 is connected with a firstelectrode contact 33 by means of a measuring electrode strip conductor27 and the reference electrode 26 is connected with a second electrodecontact 34 by means of a reference electrode strip conductor 28. Theelectrode contacts are respectively located on a front face 36 formed bythe open end of the solid electrolyte body 23. A porous protective layer29 has been placed over the measuring electrode 25 and partially overthe measuring electrode strip conductor 27. The strip conductors 27, 28are advantageously constructed as cermet layers and co-sintered.

The sensor element 14 projecting out of the longitudinal bore 18 of thehousing 11 on the side of the gas to be measured is surrounded by aspaced-apart protective pipe 44, which has openings 45 for the gas to bemeasured to enter and leave and which is fastened on the end of thehousing toward the gas to be measured. The interior of the sensorelement 14 is filled by a rod-shaped heating element 40, for example,which is fastened in a manner not shown remote from the gas to bemeasured and is provided with line connections.

A first contact element 38 rests on the first electrode contact 33 and asecond contact element 39 rests on the second electrode contact 34. Thecontact elements 38, 39 are shaped in such a way that they rest againstthe pipe-shaped heating element and are connected with a measuringelectrode connector 41 and a reference electrode connector 42. Theconnectors 41, 42 are connected with connecting cables, not shown, andare conducted to the outside to a measuring or control device.

An insulating sleeve 43 is furthermore inserted into the longitudinalbore 18 of the housing 11 and in a preferred manner consists of aceramic material. The insulating sleeve 43 is pressed on the contactelements 38, 39 by a mechanical means, not shown, by means of which theelectrical connection with the electrode contacts 33 and 34 is created.

To provide an electrically insulating and gas-tight fastening of thesensor element 14 in the housing 11, the shoulder 16 formed on thebulge-shaped head 15 is seated by means of the sealing ring 20 on thehousing 11. Metal or graphite are particularly suited as the materialfor the sealing ring 20 for sealing the interior of the sensor element14. Because of their great density, these materials are particularlyimpermeable to gas, water and motor fuel. A steel sealing ring with, forexample, a copper coating of 10 micrometers or a nickel coating of 20micrometers is practical.

A clearer representation of the sealing zone X between the sensorelement 23 and the housing 11 ensues from the respective FIGS. 2 to 4.However, a prerequisite for the employment of an electrically conductingsealing ring 20 is that the sensor element 14 is potential-free inrespect to the metal housing 11. For this purpose the strip conductor 27in a first exemplary embodiment in FIG. 2 is covered with anelectrically insulating layer 21, particularly in the area of thesealing face 22 on the side toward the sensor. The insulating layer 21has a layer thickness of 20 to 100 micrometers. In the instant exemplaryembodiment the insulating layer 21 is drawn over the entire area of thestrip conductor 27 and around the circumference of the solid electrolytebody 23 which adjoins the housing 11. However, it is also conceivable tolimit the insulating layer 21 only to the area of the sealing face 22,or to extend the insulating layer 21 on the side of the gas to bemeasured as far as the protective layer 29, which is advantageousbecause it is possible by means of this to prevent shunts, caused bydeposits of soot and/or other conducting materials from the exhaust gas,if the protective layer is sufficiently electrically insulated, forexample plasma-sprayed Mg-spinel.

A further exemplary embodiment in accordance with FIG. 3 consists incovering the strip conductors 27 with an intermediate layer 30,preferably of the material of the solid electrolyte body, and to placethe insulating layer 21 in accordance with the already describedexemplary embodiment over the intermediate layer 30, wherein theintermediate layer is suitably also co-sintered. Here the intermediatelayer 30 has the function that the glass-forming material of theinsulating layer 21 is not diffused into the material of the stripconductor 27 and in this way affects the conductivity of the stripconductor 27.

The material of the insulating layer 21 is selected in such a way thatit withstands the pressure forces of the sealing ring 20 which occurwhen joining the sensor element 14 to the housing 11, and thatfurthermore it will tolerate temperatures of at least up to 700° C. inthe area of the joint. This is achieved in that a crystalline,non-metallic material which is homogeneously distributed forms a bearingsupport frame in a glaze layer and the transformation temperature of theglass phase lies above the application temperature.

The specific electrical resistance of the crystalline, non-metallicmaterial advantageously has at least ten times the value of the specificelectrical resistance of the solid electrolyte body. Usable materialsare: Al₂ O₃, Mg-spinel, forsterite, stabilized ZrO₂, CaO- and/or Y₂ O₃-stabilized ZrO₂ with small stabilizer contents, advantageously withmaximally 2/3 of the stabilizer oxide of the full stabilization,non-stabilized ZrO₂ or HfO₂ or a mixture of these materials.

An earth alkali silicate, for example Ba-Al silicate, is used as theglass-forming material. The Ba-Al silicate has a thermal expansioncoefficient of, for example, ≧8.5×10⁻⁶ K⁻¹. The barium can be replacedby strontium up to 30 atom %.

The earth alkali silicate can be introduced as a pre-melted glass fritor as a glass-phase raw material mixture, wherein the latter isadvantageously relieved to the greatest part of water ofcrystallization, carbonate or similar annealing losses in a calcinationprocess. A small portion (<10 weight-%) of a glass-forming mixture ofraw materials is advantageously added to the glass frit. The materialmixture may contain electrically conducting impurities only up tomaximally 1 weight-%. This relates in particular to Na₂ O, K₂ O, Fe₂ O₃,TiO₂, Cu₂ O or similar semi-conducting oxides. The content ofelectrically conducting impurities lies advantageously below 0.2weight-%.

A third exemplary embodiment ensues from FIG. 4, wherein a cover layer31 is disposed over the electrically insulating layer 21 in the area ofthe sealing layer 22 on the side toward the sensor element, so that thesealing ring 20 rests against the cover layer 31 on the side toward thesensor element. The layers following on the side toward the sensorelement correspond to the exemplary embodiment in FIG. 1. However, it isalso conceivable to embody the layers on the side toward the sensorelement in accordance with the exemplary embodiment in FIG. 3. The coverlayer 31 is a thick ceramic layer, which preferably consists of thematerial of the solid electrolyte body 23, for example ofyttrium-stabilized ZrO₂. To create a thick layer, the flux portion ofthe ceramic base material is selected to be less than 10 percent,wherein no flux addition at all creates the thickest layer. The coverlayer 31 need not have an insulating resistance, instead it can have anoticeable electron and/or ion conductivity. In the case of electricalconductivity the cover layer 31 must not overlap the insulating layer21. The layer thickness of the cover layer 31 is advantageously between10 and 50 micrometers. It has been shown to be furthermore advantageousto adapt the thermal expansion coefficient of the cover layer 31 toapproximately ±2×10⁻⁶ K⁻¹ to the thermal expansion coefficient of thesolid electrolyte body.

Various examples for the composition and production of the insulatinglayer 21 and the cover layer 31 will be described below:

Example 1

Composition of the inorganic raw material mixture:

60 weight-% of alumina (99.5 weight-% of Al₂ O₃, 0.1 weight-% of Na₂ O),specific surface 15 m² /g

40 weight-% of Ba--Al silicate glass powder (53 weight-% of BaO, 5weight-% of Al₂ O₃, 42 weight-% of SiO₂, specific surface 5 m² /g).

The raw materials are homogenized and ground open over two hours in aball mill with 90% Al₂ O₃ grinding balls. Afterwards an aqueous slip isprepared with 500 g of a raw material mixture of alumina and Ba--Alsilicate glass, 500 ml of distilled water and 25 ml of an aqueouspolyvinyl alcohol solution. The slip is ground in a ball mill with 90%Al₂ O₃ grinding balls over a grinding period of 1.5 hours.

The slicker is applied by brushing to the solid electrolyte body 23,which was pre-sintered at 1000° C., of partially stabilized ZrO₂ (5mol-% of Y₂ O₃) in the area of the insulating layer 21 in accordancewith FIG. 1. Following this, the slip is co-sintered together with thesolid electrolyte body 23 for approximately 3 hours at 1450° to 1500°C., so that the insulating layer in accordance with FIG. 1 is formed.For mounting the measuring sensor, the sensor element 14 is placed onthe sealing ring 20. In this embodiment the insulation resistance at asealing ring temperature of 500° C. lies above 300 kOhm. By comparison,the insulating resistance of a sensor element 14 which was only providedwith a coating of ZrO₂ partially stabilized with 5 mol-% of Y₂ O₃ in thearea of the sealing zone 22, lies below 5 kOhm at a sealing ringtemperature of 500° C.

Example 2

This example differs in respect to the raw material mixture in Example 1in that in place of 40 weight-% of Ba--Al silicate glass powder thefollowing composition was selected:

38 weight-% of Ba--Al silicate glass powder,

1 weight-% of kaolin,

1 weight-% of barium carbonate (BaCO₃, chemically pure),

Insulation resistance>300 kOhm.

Example 3

This composition of the raw material mixture differs in respect toExample 1 in that in place of the Ba--Al silicate glass powder thefollowing components were employed:

40 weight-% of a calcinate of:

11 weight-% of kaolin,

34 weight-% of quartz (99% of SiO₂) and

55 weight-% of BaCO₃ (chemically pure)

The components are ground up for two hours in a ball mill with 90% Al₂O₃ grinding balls and are calcined as bulk material in corundum capsulesin an oxidizing atmosphere at 1000° C. for two hours and subsequentlyground open again as mentioned.

Insulation resistance>300 kOhm.

Example 4

The composition of the raw material mixture differs from that of Example1 and Example 3 as follows:

70 weight-% of alumina and 30 weight-% of calcinate,

Insulation resistance>300 kOhm.

Example 5

As in Example 4, however, in place of alumina with:

70 weight-% of partially stabilized ZrO₂ with 3.5 weight-% of MgO (35%monoclinal),

Specific surface 7 m² /g

Insulation resistance>20 kOhm.

Example 6

As in Example 3, but with:

50 weight-% of alumina,

50 weight-% of calcinate,

Insulation resistance>300 kOhm.

Example 7

As in Example 3, but with:

85 weight-% of alumina,

15 weight-% of calcinate,

Insulation resistance>200 kOhm.

Example 8

The composition of the raw material mixture corresponds to Example 6.However, in this case the slip is sprayed by means of a glazing pistolon the solid electrolyte body finished by dense sintering at 1450° to1500° C. The insulating layer is subsequently sintered in over two hoursat 1300° to 1350° C. in an oxidizing atmosphere.

Insulation resistance>100 kOhm.

Example 9

The composition corresponds to Example 7, wherein the alumina herecontains the following components:

99.3% of Al₂ O₃, 0.3% of Na₂ O,

Specific surface 2.5 m² /g,

Insulation resistance>100 kOhm.

Example 10

The composition corresponds to Example 6, however, the followingcomponents in place of alumina:

50 weight-% of monoclinal zircon oxide powder without addition of astabilizer (99.5% of ZrO₂ +HfO₂)

Specific surface 8.5 m² /g

Insulation resistance>100 kOhm.

Example 11

The composition corresponds to Example 6, however, with the followingcomponents in place of alumina:

60 weight-% of Mg-spinel powder (MgOAl₂ O₃) with <0.5 weight-% of freeMgO and <0.1 weight-% of Na₂ O,

Specific surface 8 m² /g,

Insulation resistance>300 kOhm.

Example 12

The application of the insulating layer 21 of the solid electrolyte body23 takes place as described in Example 1. The insulating layer 21 isdried at, for example 120° C., in a forced-air oven for approximatelyone hour. After this the cover layer 31 of partially stabilized zirconoxide with 5 mol-% of Y₂ O₃ is applied. Spray suspensions or pressurepastes, known per se from the prior art, are used for producing thecover layer 31, wherein the cover layer 31 is brushed on in the instantexample. Finally, the solid electrode body 23 with the electrodes andthe electrode strip conductors, the insulating layer 21 and the coverlayer 31 is co-sintered for three hours at 1450° to 1500° C.

Example 13

The production of the insulating layer 21 takes place as in Example 12,but pre-sintering of the solid electrolyte body 23 and the insulatinglayer 21 is performed at approximately 1000° C. in place of the dryingprocess. Subsequently the cover layer 31 is applied and co-sintering inaccordance with Example 12 is performed.

Example 14

The production takes place in accordance with Example 13, however, inthis case the insulating layer 21 consists of 50 weight-% of alumina and50 weight-% of Ba--Al silicate powder.

Example 15

The insulating layer 21 consists of the material in accordance withExample 1. After applying the insulating layer 21, co-sintering isperformed. This is followed by the application by means of a flamespraying method of the cover layer 31 made of fosterire powder.Subsequently tempering is performed over two hours at 1300° C.

Example 16

The production of the insulating layer 21 takes place in accordance withExample 15. In this case the cover layer 31 consists of magnesium spineland is applied by means of a flame spraying method without subsequenttempering. In this case the layer thickness of the cover layer 31 issuitably selected to be 10 micrometers.

Example 17

In this case the composition of the raw material mixture corresponds toExample 6. The slip is sprayed on in accordance with Example 8 by meansof a glazing pistol on the solid electrolyte body finished by sinteringat 1450° to 1500° C. The insulating layer 21 is thereafter sintered inover two hours at 1300° C. This is followed by producing the cover layer31 in accordance with Example 16.

We claim:
 1. An electro-chemical measuring sensor for determining theoxygen content of gases, including a potential-free arranged sensorelement having an oxygen-ions-conducting solid electrolyte body andelectrodes with electrically conducting connectors, wherein the sensorelement is inserted with a sealing ring into a metal housing and atleast one electrically conducting connector facing the housing iselectrically insulated in respect to the housing by an electricallyinsulating layer in the area of the sealing ring, wherein the insulatinglayer is formed of a mixture of a crystalline, non-metallic material anda glass-forming material such that a glaze filled with crystalline,non-metallic material is formed by heating, and wherein theglass-forming material is an earth alkali silicate glass.
 2. A measuringsensor in accordance with claim 1, wherein one of the two materialsrespectively constitutes at least 10 vol.-% of the mixture.
 3. Ameasuring sensor in accordance with claim 1, wherein the crystalline,non-metallic material is selected from the group consisting of Al₂ O₃,Mg-spinel, forsterite, MgO-stabilized ZrO₂, CsO- and/or Y₂ O₃-stabilized ZrO₂, non-stabilized ZrO₂, HfO₂ and mixtures thereof.
 4. Ameasuring sensor in accordance with claim 1, wherein the insulatinglayer has a thermal expansion coefficient which is at leastapproximately matched to the thermal expansion coefficient of thematerial of the solid electrolyte body.
 5. A measuring sensor inaccordance with claim 4, wherein the crystalline, non-conductingmaterial has a thermal expansion coefficient of greater than 6×10⁻⁶ K⁻¹.6. A measuring sensor in accordance with claim 1, wherein theglass-forming material is a barium-aluminum silicate glass.
 7. Ameasuring sensor in accordance with claim 6, wherein up to 30 atom-% ofbarium are substituted by strontium.
 8. A measuring sensor in accordancewith claim 1, wherein the electrically insulating layer is placed aroundthe solid electrolyte body at least in the area of the sealing ring. 9.A measuring sensor in accordance with claim 1, wherein the insulatinglayer extends up to a protective layer covering the measuring electrode.10. A measuring sensor in accordance with claim 1, wherein the layerthickness of the insulating layer is 10 to 100 micrometers.
 11. A methodfor producing a potential-free disposed sensor element for a measuringsensor in accordance with claim 1, wherein the mixture of the insulatinglayer, consisting of the crystalline, non-conducting material and theglass-forming material, is subjected to a thermal treatment above themelting temperature of the glass-forming material.
 12. A method inaccordance with claim 11, wherein the glass-forming material isintroduced into the mixture in the form of a pre-melted glass frit. 13.A method in accordance with claim 12, wherein the glass frit is usedwith an addition of a glass-forming raw material mixture less than 10%.14. A method in accordance with claim 11, wherein the glass-formingmaterial is introduced into the mixture in the form of a mixture ofglass-forming raw materials.
 15. A method in accordance with claim 11,wherein the glass-forming raw materials are relieved of water ofcrystallization, carbonate and similar heating losses in a calcinationprocess in a proportion of greater than 90%.
 16. A method in accordancewith claim 11, wherein the thermal treatment of the insulating layer isperformed by co-sintering with the solid electrolyte body.
 17. Ameasuring sensor in accordance with claim 1 wherein the solidelectrolyte body is in the form of a pipe closed at one end.
 18. Anelectro-chemical measuring sensor for determining the oxygen content ofgases including a potential-free arranged sensor element having anoxygen-ions-conducting solid electrolyte body and electrodes withelectrically conducting connectors, wherein the sensor element isinserted with a sealing ring into a metal housing and at least oneelectrically conducting connector facing the housing is electricallyinsulated with respect to the housing by an electrically insulatinglayer in the area of the sealing ring, wherein the insulating layer isformed of a mixture of a crystalline, non-metallic material and aglass-forming material such that a glaze filled with the crystalline,non-metallic material is formed by heating, and wherein an intermediatelayer is disposed between the electrically conducting connector and theinsulating layer at least in the area of the electrically insulatingsection.
 19. A measuring sensor in accordance with claim 18, wherein theintermediate layer consists of the material of the solid electrolytebody.
 20. An electro-chemical measuring sensor for determining theoxygen content of gases including a potential-free arranged sensorelement having an oxygen-ions-conducting solid electrolyte body andelectrodes with electrically conducting connectors, wherein the sensorelement is inserted with a sealing ring into a metal housing, wherein atleast one electrically conducting connector facing the housing iselectrically insulated with respect to the housing by an electricallyinsulating layer in the area of the sealing ring, wherein the insulatinglayer is formed of a mixture of a crystalline, non-metallic material anda glass-forming material such that a glaze with the crystalline,non-metallic material is formed by heating, and wherein a ceramic coverlayer is placed over the insulating layer, at least in the area of thesealing ring, to absorb mechanical pressure forces of the sealing ring.21. A measuring sensor in accordance with claim 20, wherein the coverlayer is a dense ceramic layer, to whose material a flux agent of lessthan 10% has been added prior to sintering.
 22. A measuring sensor inaccordance with claim 21, wherein the material of the cover layerconsists of the material of the solid electrolyte body.
 23. A measuringsensor in accordance with claim 20, wherein the thickness of the coverlayer is 10 to 50 micrometers.